The fundamental purpose of these studies is to define the mechanisms by which peptide growth factors stimulate replication of human fibroblasts and to determine if and how these processes are altered during aging. Several older adult donor fibroblast strains will be used to compare their production rates of a fibroblast somatomedin-like peptide to that of several younger donor strains. The effect of age on the response to human growth hormone and platelet derived growth factor will be determined. Cell fusion techniques will be used to determine if a cytoplasmic or nuclear signal is responsible for the age-related decline in fibroblast somatomedin production. Since the synthesis and secretion of fibroblast somatomedin may be important steps in controlling fibroblast replication, a model system will be developed to study the biosynthesis of this peptide. The size of the synthetic product(s) will be analyzed using both younger and older donor fibroblasts to determine if there are age related changes. The fibroblast somatomedin-like peptide will be isolated in pure form and its activity determined using several bioassay systems. The potency of this peptide in stimulating human fibroblast replication will be compared to that of pure somatomedin-C. The hormones and growth factors which enhance the effect of fibroblast somatomedin will be determined and where specific additive effects can be delineated, the effect of these combinations on fibroblasts from older donors will be determined. Since fibroblast somatomedin appears to be required for replication, anti-somatomedin-C antibodies will be used to determine the point in the cell cycle at which these cells undergo growth arrest. Since human fibroblasts from older donors have a significant decrease in somatomedin-C receptor affinity the older donor fibroblast receptor will be analyzed for changes in specificity and/or changes in structure using affinity crosslinking techniques. A newly described growth factor which is secreted only by embryonic fibroblasts will be purified and its biologic effects determined. The results of these studies should help to substantiate the hypothesis that human fibroblasts secrete a somatomedin-like peptide that modulates their rates of replication. Analysis of the regulatory events that control this process should provide a basis for designing studies to investigate the etiology of the diminished replication of older donor fibroblast cultures.